1. Field of the Invention
The present invention relates to an optical waveguide device having a photonic crystal structure.
2. Description of the Related Art
It is known that, when a structure whose refractive index is modulated with a period on the order of a light wavelength is formed in a substrate of a material which is capable of transmitting electromagnetic waves (including light), e.g., a dielectric or semiconductor, a band structure, which is similar to an electronic band structure in a crystal, will appear in the dispersion relation for propagation of electromagnetic waves. Such a band with respect to electromagnetic waves (typically “light”) is referred to as a photonic band, and a periodic structure which has a photonic band is referred to as a “photonic crystal”. A known photonic crystal is described in J. D. Joannopouls et al. “Photonic crystals”, Princeton University Press, 1995.
Optical waveguide devices utilizing photonic crystals are regarded as being a promising technique for downsizing optical circuit devices. What is important in an optical waveguide device in which photonic crystals are used to confine light in a predetermined region is the difference in refractive index between an optical waveguide portion and photonic periodic structures. Therefore, there have been reported examples of combining a material whose refractive index is three or more, e.g., gallium arsenide (refractive index: 3.6) or silicon (refractive index: 3.4), and a low refractive index material, e.g., silicon dioxide (SiO2; refractive index: 1.5) or air (refractive index: 1) (see, for example, Japanese Laid-Open Patent Publication No. 2002-350657).
Among others, there has been proposed an optical waveguide (Chutinan et. al., Physical Review B, vol. 62, No. 7, p 4488 2000) in which periodic cylindrical air holes are provided in a semiconductor substrate to create a photonic band, thus making it possible to deflect an optical path by a sharp angle of 90°.
FIG. 1 shows an example of an optical waveguide device utilizing a photonic crystal structure (Chutinan et. al., Physical Review B, vol. 62, No. 7, p 4488 2000). This optical waveguide device is produced by forming a periodic array of many air holes 102 on a substrate 101, which is formed of InP or GaAs. A linear defect portion 103, in which no air holes 102 are present, has no photonic band therein and is able to transmit light in a broad range of wavelengths.
Thus, by providing photonic crystal regions on both sides of an optical waveguide, it becomes possible, with the action of the band structures in the photonic crystal regions, to confine within the optical waveguide any light of a wavelength of which transmission is to be prohibited.
With such an optical waveguide device, it is possible to realize a optical waveguide which is bent at a sharp angle that cannot be attained by a conventional refractive-index-type optical waveguide device, and there are aspirations towards application to optical circuit devices which are on the wavelength order. As for the optical waveguide device shown in FIG. 1, many applications have been proposed, such as an optical integrated circuit in which optical filters, semiconductor lasers, and the like are integrated.
Moreover, U.S. Pat. No. 6,853,791 discloses an optical waveguide device utilizing a slab-like photonic crystal which is formed by forming air holes at triangular lattice points. In this optical waveguide device, the size and shape of air holes at specific lattice point positions in the triangular lattice are made different from the size and shape of the air holes in other lattice point positions, thus forming a line defect region which functions as an optical waveguide.
However, the optical waveguide device shown in FIG. 1 has a problem in that the transmittance becomes lower than expected. This problem will by described by referring to FIG. 2.
FIG. 2 shows a plan view of an optical waveguide which is interposed between photonic crystal regions, and an enlarged view of a portion thereof.
According to a study by the inventors, the cause for the lowered transmittance is the effective refractive index of the optical waveguide being changed periodically along the waveguiding direction by air holes 202 and 203 which are arranged in a periodic array for forming a photonic crystal structure. Specifically, in a substrate 201, the effective refractive index of each first portion 204 which is interposed between two air holes 202 and 203 becomes lower than the effective refractive index of any second portion 205 which is interposed between two adjoining first portions 204. Therefore, inside the optical waveguide, the effective refractive index is periodically changed along the waveguiding direction, such that the optical waveguide characteristics will exhibit resonator-like properties. As a result of this, some of the light propagating through the optical waveguide resonates, and thus the characteristics of the optical waveguide are deteriorated due to reflection or undesirable band characteristics.
In the optical waveguide device disclosed in U.S. Pat. No. 6,853,791, air holes of e.g. an elliptical shape are formed in positions corresponding to the second portions 205 in FIG. 2. By adopting such a structure, the effective refractive index of the second portions 205 is greatly lowered than the refractive index of the substrate material, which means that the effective refractive index of the first portions 204 will be higher than the effective refractive index of the second portions 205. Therefore, the problem of resonator-like properties appearing in the optical waveguide characteristics is not solved by the waveguide device of U.S. Pat. No. 6,853,791, either.